A typical hydrostatically driven vehicle includes an engine having an output shaft connected to one or more fluid pumps. A variable displacement fluid pump is connected to the engine through a transmission and varies its displacement to accommodate power demands of various vehicle systems. The fluid pump or pumps associated with the vehicle is connected via fluid pressure lines to various actuators and hydraulic motors located on the vehicle. For example, a vehicle may have one or more hydraulic propel motors that rotate the drive wheels or rollers and move the vehicle along a base surface. Additionally, the vehicle may have one or more implements performing various functions. In the case of an asphalt or soil compactor, the implement may be a vibratory system operably coupled to the compacting drum of the vehicle. The vibratory system may operate to impart compacting energy to the base surface, thus increasing the efficiency of compaction. The vibratory system may include a hydraulic motor powered by pressurized flow of fluid from an implement pump. The speed, torque output, and power consumption of these motors are proportional to the flow rate and pressure of the hydraulic fluid passing through them.
An engine on a hydrostatically driven vehicle often typically operates in a continuous fashion at a constant speed and fueling command or, stated differently, at a constant power output. Even though the power output of the engine is constant, the power used or consumed by the various hydraulic systems of the vehicle may change based on operator commands or environmental conditions. For example, a vehicle will consume more power when travelling at a faster speed, or may impart more energy to the surface being compacted when the surface is wet.
The flow rate of hydraulic fluid supplied to the various system motors of the vehicle may be modulated to control the power supplied to those systems. Modulation may be accomplished, for example, through control of displacement pumps used in the systems or by selectively routing fluid to the various motors and actuators of the vehicle. In general, however, most vehicles operating under most conditions do not entirely consume the power generated by their engines. As is often the case, an appreciable percentage of the power generated by the engine is unused or wasted. This mode of operation causes reduced fuel economy during operation.
U.S. Patent Application Publication No. 2009/0143952 to Chisholm et al. proposes to address fuel economy concerns by estimating the current power consumption of the vehicle, comparing it to the current power output of the engine, and adjusting the engine operating parameters to more closely match the estimated power consumption. Sensors are used to detect the fluid pressure at various points in the hydraulic circuit, which data are then used to estimate the current power consumption of the vehicle.